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Change statement to

  DBCC CHECKIDENT('TableName', RESEED, 1)

This will start from 2 (or 1 when you recreate table), but it will never be 0.

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最佳答案

As you pointed out in your question it is a documented behavior. I still find it strange though. I use to repopulate the test database and even though I do not rely on the values of identity fields it was a bit of annoying to have different values when populating the database for the first time from scratch and after removing all data and populating again.

A possible solution is to use truncate to clean the table instead of delete. But then you need to drop all the constraints and recreate them afterwards

In that way it always behaves as a newly created table and there is no need to call DBCC CHECKIDENT. The first identity value will be the one specified in the table definition and it will be the same no matter if you insert the data for the first time or for the N-th

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This is documented behavior, why do you run CHECKIDENT if you recreate the table, in that case skip the step or use TRUNCATE (if you don't have FK relationships)

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You are right in what you write in the edit of your question.

After running DBCC CHECKIDENT('TableName', RESEED, 0):
- Newly created tables will start with identity 0
- Existing tables will continue with identity 1

The solution is in the script below, it's sort of a poor-mans-truncate :)

-- Remove all records from the Table
DELETE FROM TableName


-- Use sys.identity_columns to see if there was a last known identity value
-- for the Table. If there was one, the Table is not new and needs a reset
IF EXISTS (SELECT * FROM sys.identity_columns WHERE OBJECT_NAME(OBJECT_ID) = 'TableName' AND last_value IS NOT NULL)
DBCC CHECKIDENT (TableName, RESEED, 0);

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I did this as an experiment to reset the value to 0 as I want my first identity column to be 0 and it's working.

dbcc CHECKIDENT(MOVIE,RESEED,0)
dbcc CHECKIDENT(MOVIE,RESEED,-1)
DBCC CHECKIDENT(MOVIE,NORESEED)

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USE AdventureWorks2012;
GO
DBCC CHECKIDENT ('Person.AddressType', RESEED, 0);
GO






AdventureWorks2012=Your databasename
Person.AddressType=Your tablename

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I have used this in SQL to set IDENTITY to a particular value:-

DECLARE @ID int = 42;
DECLARE @TABLENAME  varchar(50) = 'tablename'


DECLARE @SQL nvarchar(1000) = 'IF EXISTS (SELECT * FROM sys.identity_columns WHERE OBJECT_NAME(OBJECT_ID) = '''+@TABLENAME+''' AND last_value IS NOT NULL)
BEGIN
DBCC CHECKIDENT('+@TABLENAME+', RESEED,' + CONVERT(VARCHAR(10),@ID-1)+');
END
ELSE
BEGIN
DBCC CHECKIDENT('+@TABLENAME+', RESEED,' + CONVERT(VARCHAR(10),@ID)+');
END';
EXEC (@SQL);

And this in C# to set a particular value:-

SetIdentity(context, "tablename", 42);
.
.
private static void SetIdentity(DbContext context, string table,int id)
{
string str = "IF EXISTS (SELECT * FROM sys.identity_columns WHERE OBJECT_NAME(OBJECT_ID) = '" + table
+ "' AND last_value IS NOT NULL)\nBEGIN\n";
str += "DBCC CHECKIDENT('" + table + "', RESEED," + (id - 1).ToString() + ");\n";
str += "END\nELSE\nBEGIN\n";
str += "DBCC CHECKIDENT('" + table + "', RESEED," + (id).ToString() + ");\n";
str += "END\n";
context.Database.ExecuteSqlCommand(str);
}

This builds on the above answers and always makes sure the next value is 42 (in this case).

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It seems ridiculous that you can't set/reset an identity column with a single command to cover both cases of whether or not the table has had records inserted. I couldn't understand the behavior I was experiencing until I stumbled across this question on SO!

My solution (ugly but works) is to explicitly check the sys.identity_columns.last_value table (which tells you whether or not the table has had records inserted) and call the appropriate DBCC CHECKIDENT command in each case. It is as follows:

DECLARE @last_value INT = CONVERT(INT, (SELECT last_value FROM sys.identity_columns WHERE OBJECT_NAME(OBJECT_ID) = 'MyTable'));
IF @last_value IS NULL
BEGIN
-- Table newly created and no rows inserted yet; start the IDs off from 1
DBCC CHECKIDENT ('MyTable', RESEED, 1);
END
ELSE
BEGIN
-- Table has rows; ensure the IDs continue from the last ID used
DECLARE @lastValUsed INT = (SELECT ISNULL(MAX(ID),0) FROM MyTable);
DBCC CHECKIDENT ('MyTable', RESEED, @lastValUsed);
END

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Simply do this:

IF EXISTS (SELECT * FROM tablename)
BEGIN
DELETE from  tablename
DBCC checkident ('tablename', reseed, 0)
END

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Borrowing from Zyphrax's answer ...

USE DatabaseName


DECLARE @ReseedBit BIT =
COALESCE((SELECT SUM(CONVERT(BIGINT, ic.last_value))
FROM sys.identity_columns ic
INNER JOIN sys.tables t ON ic.object_id = t.object_id), 0)
DECLARE @Reseed INT =
CASE
WHEN @ReseedBit = 0 THEN 1
WHEN @ReseedBit = 1 THEN 0
END


DBCC CHECKIDENT ('dbo.table_name', RESEED, @Reseed);

Caveats: This is intended for use in reference data population situations where a DB is being initialized with enum type definition tables, where the ID values in those tables must always start at 1. The first time the DB is being created (e.g. during SSDT-DB publishing) @Reseed must be 0, but when resetting the data i.e. removing the data and re-inserting it, then @Reseed must be 1. So this code is intended for use in a stored procedure for resetting the DB data, which can be called manually but is also called from the post-deployment script in the SSDT-DB project. In that way the reference data inserts are only defined in one place but aren't restricted to be used only in post-deployment during publishing, they are also available for subsequent use (to support dev and automated test etc.) by calling the stored procedure to reset the DB back to a known good state.